Illuminator and method for manufacturing the illuminator

ABSTRACT

To provide a reflection type liquid-crystal display device having an illuminator in front of the liquid-crystal display device, the illuminator having a light guide plate superior in light utilization efficiency, dots constituted by a plurality of small recess or protrusion portions for reflecting the light incident on the light guide plate toward the liquid-crystal display device are formed in or on a surface of the light guide plate which does not face the liquid-crystal display device. Each of the dots is set to have a substantially V-shape in section, the sectional inclination angle of the V-shape is set in a range of from 35 to 43°, and the sectional vertex angle of the V-shape is in a range of 70.6±2.5°.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an illuminator provided with alight guide plate, and to a liquid-crystal display device using theilluminator.

[0002] Recently, portable electronic apparatuses represented by aportable information terminal, a portable telephone, etc. have been madesmaller in size and lower in price. As a result, such portableelectronic apparatuses have come into wide use.

[0003] Specific examples of such a portable electronic apparatus includea reflection type liquid-crystal display device which is effective inreducing power consumption, and a liquid-crystal display device using areflection type liquid-crystal display element and a front-lightingilluminator.

[0004] As the performance required of this illuminator, the irradiationquantity of the light which irradiates a liquid-crystal panel has to belarge and the whole surface of the liquid-crystal panel has to beirradiated uniformly. The enhancement of the irradiation quantity oflight is achieved easily by the increase of the quantity of the lightradiated from a light source. However, such a method cannot be regardedas practical because it is accompanied by the increase of powerconsumption.

[0005] As the background art concerning such an illuminator,JP-A-10-188636 discloses a method in which a light source is disposed inan end portion of a light guide plate made of a transmissive materialand small protrusion portions for taking the light out toward aliquid-crystal display device are formed on the lower surface of thelight guide plate, as shown in FIG. 1.

[0006] In addition, JP-A-11-53918 discloses another method in whichsmall protrusion portions (or small recess portions) for reflecting thelight entering a light guide plate toward a liquid-crystal displaydevice are formed on the upper surface of the light guide plate, asshown in FIG. 1.

[0007] Further, JP-A-11-72787 discloses a further method in which smallprotrusion portions or small recess portions for transmitting the lightentering a light guide plate toward a liquid-crystal display device areformed at random on the lower surface of the light guide plate.

SUMMARY OF THE INVENTION

[0008] The following properties are required of an illuminator for usein the condition that it is disposed in front of a liquid-crystaldisplay device.

[0009] (1) Haze (turbidity or cloudiness) of a light guide plate is low.

[0010] (2) The surface reflectivity is low.

[0011] (3) The light entering eyes directly from the light guide plateused in the illuminator is less.

[0012] (4) The exiting angle of the light made to go out from the lowersurface of the light guide plate used in the illuminator is small.

[0013] (5) No moire pattern is produced.

[0014] In the above-mentioned background art disclosed inJP-A-10-188636, however, the exiting angle of the light made to go outfrom the lower surface of the light guide plate becomes large due to thesectional shape of each of the protrusion portions. Therefore, there issuch a problem that not only is the light entering the liquid-crystaldisplay device less, but also the protrusion portions are arrangedregularly so that moire is produced easily.

[0015] On the other hand, in the background art disclosed inJP-A-11-53918, the aforementioned problem can be improved somewhat, butthe light entering eyes directly from the light guide plate is apt to begenerated due to the shape of each of the protrusion portions (or recessportions). As a result, a problem in visibility as a liquid-crystaldisplay device still remains. In addition, moire is produced easily dueto the arrangement of the protrusion portions.

[0016] Further, in the background art disclosed in JP-A-11-72787, theexiting angle of the light made to go out from the lower surface of thelight guide plate becomes large due to the sectional shape of each ofthe protrusion portions. Accordingly, not only is the light entering theliquid-crystal display device less, but also it is difficult to disposea large number of small protrusion portions or small recess portionsirregularly.

[0017] The present invention has been developed to solve the foregoingproblems. It is an object of the present invention to provide anilluminator which can enhance the irradiation quantity of the lightwhich irradiates a liquid-crystal display device without increasing thequantity of the light radiated from a light source; a method formanufacturing the illuminator; and a liquid-crystal display device usingthe illuminator.

[0018] In order to attain the foregoing object, an illuminator disposedin front of a liquid-crystal cell according to the present invention isconstituted by a light guide plate and a light source disposed on one ofside surfaces of the light guide plate. This light guide plate includesan incidence surface on which the light from the light source isincident, and a light transmission surface through which the incidentlight on the light guide plate is made to exit to the liquid-crystalcell. In addition, a plurality of dots each constituted by a smallrecess portion or a small protrusion portion for reflecting the lightincident on the incidence surface toward the light transmission surfaceare formed on the surface opposite to the light transmission surface.Each of the dots has a substantially V-shape in section, and aninclination angle of the section is in a range of from 35 to 43°. Avertex angle of each of the dots is set to be in a range of 70.6±2.5°.

[0019] In addition, according to the present invention, each of the dotsis substantially rectangular in plan shape, and each of the dots is setto have a short-side length in a range of from 0.002 to 0.05 mm and along-side length in a range of from 0.002 to 0.2 mm.

[0020] Then, the dots each having such a shape are disposed at random onthe surface opposite to the light transmission surface of the lightguide plate.

[0021] Further, according to the present invention, the light guideplate constituting the illuminator includes an incidence surface onwhich the light from the light source is incident, and a lighttransmission surface through which the incident light on the light guideplate is made to exit to the liquid-crystal cell, and a plurality ofdots each constituted by a small recess portion or a small protrusionportion for reflecting the light incident on the incidence surfacetoward the light transmission surface are formed on the surface oppositeto the light transmission surface.

[0022] Then, not smaller than 95% of the whole area of the surface onwhich the dots are formed is sectioned into 0.25 to 1 mm² square areas,and the dots are disposed in each of the square areas so that a functionG(R) which is obtained by taking a weighted average of a radialdistribution function g(R) obtained for each of the dots in accordancewith an arrangement relationship of the dots, and which is obtained byapproximating the weighted average by a least squares method satisfies arelation of 0<S₁/S₂<0.2 in a range of R/R₀=3 to 6.

[0023] Provided that R designates a distance from a central position ofone dot to a central position of another dot; R₀, a value obtained bydividing a length of one side of the square area by a square root of thenumber of the dots existing in the square area; S₁, a value obtained byintegrating a difference between G(R) and an average value of G(R) withR/R₀ which is in a range of from 3 to 6; and S₂, a value obtained byintegrating the average value of G(R) with R/R₀ which is in a range offrom 3 to 6.

[0024] In addition, each of the dots is disposed so that the functionG(R) is substantially 0 in a range of R<(short-sidle length of dot)×2,at least two peaks exist in the function G(R), and two peaks each ofwhich is at least twice as large as the average value of the functionG(It) exist in a range of R/R₀=3 to 6.

[0025] Moreover, according to the present invention, an oxide film isformed on a surface of a silicon substrate having a predeterminedcrystal plane and a resist film is formed on the oxide film so that eachdot to be formed on the light guide plate has a V-shape in section andan inclination angle of the section is in a range of from 35 to 43°.Then, a dot pattern is formed on the oxide film with the resist filmserving as a mask. Then, after anisotropic etching is given to thesilicon substrate with the oxide film serving as a mask, a metal film isfurther formed on the silicon substrate. Further, the metal film isstripped off so as to produce a stamper or a replica thereof. Dots aretransferred onto a surface of a film or plastic by use of the stamper orthe replica. Thus, a light guide plate having such dots is formed.

[0026] A liquid-crystal display device according to the presentinvention has an illuminator which has such a light guide plate, aliquid-crystal display portion and a control portion. The illuminator isdisposed in front of the liquid-crystal display portion so that externallight is transmitted through the illuminator and enters theliquid-crystal display portion. The quantity of light with which theilluminator irradiates the liquid-crystal display portion is controlledby the control portion in accordance with the quantity of the externallight.

[0027] Further, a portable electronic apparatus using the liquid-crystaldisplay device according to the present invention has a light receivingportion. The illuminator is controlled by use of the quantity of theexternal light received by the light receiving portion so that theluminance of the liquid-crystal display portion is made substantiallyconstant.

[0028] Furthermore, the portable electronic apparatus also has a signalreceiving portion. The illuminator is controlled by the control portionwith a signal supplied to the signal receiving portion as a trigger, sothat the liquid-crystal display portion is irradiated with light inaccordance with the external light entering the light receiving portion.Thus, the luminance of the liquid-crystal display portion is madesubstantially constant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

[0030]FIG. 1 is a perspective view of an illuminator for explaining afirst embodiment;

[0031]FIG. 2 is a sectional view of the illuminator for explaining thefirst embodiment;

[0032]FIG. 3 is a sectional view and a plan view of a micro-dot (smallrecess portion);

[0033]FIG. 4 is a sectional view and a plan view of a micro-dot (smallprotrusion portion);

[0034]FIG. 5 is a view for explaining tracks of light propagated insidea light guide plate according to the first embodiment;

[0035]FIG. 6 is a conceptual view for explaining an arrangement ofmicro-dots;

[0036]FIGS. 7A and 7B are views for explaining sectional inclinationangles and sectional vertex angles of micro-dots respectively;

[0037]FIG. 8A is a graph showing a relationship between the sectionalinclination angle of the micro-dots and efficiency of the light made toexit from the light guide plate;

[0038]FIG. 8B is a graph showing a relationship between the sectionalinclination angle of the micro-dots and the exiting angle of the lightmade to exit from the light guide plate when the angle of the lightpropagated inside the light guide plate is in a range of ±25°;

[0039]FIG. 8C is a graph showing a relationship between the sectionalinclination angle of the micro-dots and the exiting angle of the lightmade to exit from the light guide plate when the angle of the lightpropagated inside the light guide plate is in a range of ±35°;

[0040]FIGS. 9A and 9B are views for explaining the influence of therange of the exiting angle of the light made to exit from the lightguide plate, respectively;

[0041]FIGS. 10A to 10C are views for explaining sectional shapes ofmicro-dots respectively;

[0042]FIGS. 11A to 11D are views for explaining other forms of themicro-dot respectively;

[0043]FIGS. 12A to 12C are views for explaining a method for arrangingthe micro-dots;

[0044]FIG. 13 is a view for explaining a problem in the arrangement ofmicro-dots;

[0045]FIG. 14 is a graph for explaining a distribution of a radialdistribution function G(R);

[0046]FIGS. 15A to 15G are views showing a flow of steps of a method formanufacturing a light guide plate having micro-dots, up to the step offorming a dot pattern;

[0047]FIGS. 15H to 15M are views showing the flow of steps of the methodfor manufacturing a light guide plate having micro-dots, up to the stepof producing the light guide plate by use of an etchinginjection-molding method;

[0048]FIGS. 16A and 16B are views for explaining the production of asilicon substrate having a predetermined crystal plane, respectively;

[0049]FIG. 17 is an explanatory view of the silicon substrate having thepredetermined crystal plane;

[0050]FIG. 18 is a view for explaining a liquid-crystal display devicewith an illuminator disposed in front of the display device according toa second embodiment;

[0051]FIG. 19 is a schematically sectional view of the liquid-crystaldisplay device with the illuminator disposed in front of the displaydevice;

[0052]FIG. 20 is a schematically sectional view of a portable electronicapparatus according to a third embodiment; and

[0053]FIG. 21 is a graph for explaining the relationship between thefrontal luminance of the liquid-crystal display device and the quantityof external light.

DETAILED DESCRIPTION OF EMBODIMENTS

[0054] Embodiments of the present invention will be described below indetail with reference to the drawings.

[0055]FIG. 1 is a perspective view of an illuminator for use in aliquid-crystal display device according to a first embodiment. FIG. 1also includes a perspective view of a plurality of small recess portionsor small protrusion portions (hereinafter referred to as “micro-dots”)surrounded by a light guide plate flat portion, for changing thetravelling direction of light in the light guide plate. Incidentally,FIG. 1 illustrates the case where the dots are formed into small recessportions.

[0056] A light guide plate 1 is disposed in front of a liquid-crystaldisplay device 8 so that the light from light sources 6 disposed on oneof side surfaces of the light guide plate 1 is made incident on thelight guide plate 1 and this incident light is reflected toward theliquid-crystal display device 8 by dots 5 provided on an upper surface 7of the light guide plate 1.

[0057]FIG. 2 is a sectional view conceptually showing the relationshipbetween the illuminator and the liquid-crystal display device. Asdescribed above, the light from the light sources 6 reflected by thedots 5 (the light 3 made to exit from the lower surface of the lightguide plate) enters the liquid-crystal display device 8, and isreflected by a reflection plate provided in the liquid-crystal displaydevice 8. Then, the reflected light is transmitted through the lightguide plate L again and reaches eyes of an observer.

[0058] Incidentally, of the light entering the light guide plate IL fromthe light sources 6, the light 2 shown in FIG. 2 reaches the eyes of theobserver directly without travelling through the liquid-crystal displaydevice 8. In consideration of the object of the illuminator described inthis embodiment, not to say, the light 2 reaching the eyes of theobserver directly is not preferable, and it should be reduced to theutmost.

[0059] Detailed description will be made about the shape of themicro-dots 5 for attaining the foregoing object.

[0060]FIG. 3 shows a sectional view and a plan view of a micro-dot 5which is a small recess portion by way of example. On the other hand,FIG. 4 is a sectional view and a plan view of a micro-dot 5 which is asmall protrusion portion. In addition, FIG. 5 is a view for explainingthe tracks of light propagated in the light guide plate in thisembodiment.

[0061] Description will be made about the case where each of themicro-dots 5 is a small recess portion by way of example. Theilluminator shown in FIGS. 3 and 5 is provided with light sources 6 anda light guide plate 1. In addition, as described above, the illuminatoris disposed in front of a liquid-crystal display device 8, and the lightsources 6 are disposed on one of side surfaces of the light guide plate1. The micro-dots 5 each shaped as shown in FIG. 3 are formed on anupper surface 7 of the light guide plate 1. That is, as shown in FIG. 5,the micro-dots 5 are formed on the surface (the light-guide-plate uppersurface 7) of the light guide plate 1 which does not face theliquid-crystal display device 8.

[0062] This dot arrangement is to use the micro-dots 5 to reflect thelight 9 incident on the side surface of the light guide plate 1 towardthe liquid-crystal display device 8. Specifically, in FIG. 6, the lightexiting from the light source 6 enters, as the light-guide-plateincident light 9, the light guide plate 1 through an incidence endsurface of the light guide plate 1 so as to form light-guide-plateguided light. The light-guide-plate guided light travels toward theother end surface of the light guide plate 1 while total reflection isrepeated between a light-guide-plate lower surface 10 and thelight-guide-plate upper surface 7.

[0063] Of the light-guide-plate guided light, the light 18 travelling toa reflection slope 11 of the corresponding micro-dot 5 is reflected bythe slope so as to travel toward the light-guide-plate lower surface 10.The light reaching the light-guide-plate lower surface 10 exits from thelight guide plate 1 while being refracted by the light-guide-plate lowersurface 10. The refracted light enters the liquid-crystal display device8 as illumination light.

[0064] Thus, by use of the light guide plate 1 described in thisembodiment, the light from the light source 6 can be made to exit towardthe liquid-crystal display device 8 efficiently.

[0065] At this time, when a sectional slope angle 12 of thecorresponding micro-dot 5 is selected so that the light reflected by themicro-dot reflection slope 11 satisfies the condition of totalreflection, the light 19 entering the eyes of the observer directly fromthe light guide plate 1 can be reduced. As a result, the quantity of thelight made incident from the light guide plate 1 to the liquid-crystaldisplay device 8 becomes so large that a reflection type liquid-crystaldisplay device with good visibility can be realized.

[0066] The intensity of the light made to exit from the light source 6is generally reduced in the light guide plate 1 as the distance from thelight source 6 increases. Accordingly, the intensity of the lightentering the liquid-crystal display device 8 can be uniformalized, forexample, by changing the density of the micro-dots 5, that is, bychanging the number of the micro-dots 5 per unit area, or by changingthe size or length of the micro-dots 5 while keeping the density of themicro-dots 5 uniform, or by a combination of the aforementioned methods.

[0067] In the case of a single light source, it is preferable that thedensity of the micro-dots 5 is formed to increase, in accordance withthe exponential function or power law, from the light-source-side endsurface of the light guide plate toward the opposite end surface of thelight guide plate.

[0068] The sectional shape of each of the micro-dots 5 is substantiallyperpendicular to the upper surface 7 of the light guide plate on whichthe micro-dots 5 are formed, as shown in FIG. 6. The sectional shapetaken on a plane substantially in parallel with the travelling directionof the light entering the light guide plate 1 from the light source 6(the travelling direction of the guided light in the light guide plate)is substantially a V-shape. The sectional inclination angle 12 is set tobe in a range of from 35 to 43°, and the sectional vertex angle is setto be in a range of 70.6±2.5°. Incidentally, in the case of point lightsources, it may be considered that there is a linear light sourceconnecting those point light sources.

[0069] Here, the micro-dot reflection slope 11, the sectionalinclination angle 12, dot depth (dot height) H, dot short-side lengthW15 and dot long-side length L16 are defined in FIGS. 3 to 4 and FIGS.7A and 8B. That is, as shown in FIGS. 7A and 7B, the sectionalinclination angle 12 is set to be an angle between the light-guide-plateupper surface 7 and a straight line connecting points a and b on themicro-dot reflection slope 11 when the micro-dot depth (height) H14 isdivided to three equal parts. In addition, the vertex angle 17 is set tobe an angle between the straight line connecting the points a and b anda straight line connecting points C and D on the micro-dot reflectionslope 11.

[0070] The reason why the sectional inclination angle 12 is set to be ina range of from 35 to 43° is to reduce the light 2 (see FIG. 2) enteringeyes directly from the light guide plate 1 and to increase the quantityof the light entering the liquid-crystal display device 8 so as toimprove the visibility of the liquid-crystal display device 8.

[0071] That is, as shown in FIG. 5, light with a divergent angle withinabout ±35°, more particularly within about ±25° with respect to thehorizontal direction in which the light is travelling is generallypropagated inside the light guide plate 1. The light incident on thereflection slope 11 of the micro-dot 5 is reflected and refracted by theslope 11. Of the reflected and refracted light, the reflected lightchanges its travelling direction downward and exits from thelight-guide-plate lower surface 10 so as to function as illuminationlight for the liquid-crystal display device 8.

[0072] On the other hand, the light refracted and transmitted by thereflection slope 11 of the micro-dot 5 exits from the upper surface 7 ofthe light guide plate 1 without entering the liquid-crystal displaydevice 8. Thus, the light enters the eyes of the observer. Therefore,the observer directly views the light from the light sources 6 so as toform a bright spot or a bright line, which lowers the value as aliquid-crystal display device. Further, since the aforementioned lightdoes not illuminate the liquid-crystal display device 8, the utilizationefficiency of the light from the light sources 6 is reduced.

[0073] It is therefore necessary to set the sectional inclination angle12 so that reflection by the light-source-side slope of the micro-dot 5satisfies the total reflection condition to the utmost. As a result, itis possible to utilize the light from the light sources 6 largely forilluminating the liquid-crystal display device.

[0074]FIG. 8A shows the relationship between the sectional inclinationangle and the efficiency when the light from the light sources is madeto exit from the light-guide-plate lower surface. On the other hand,FIGS. 8B and 8C show the relationship between the sectional inclinationangle and the range (half width) of the exiting angle of the light madeto exit from the light-guide-plate lower surface. Incidentally, in eachcase, the propagation angle with which light travels while reflected bythe upper and lower surfaces of the light guide plate was varied within±35°, and more particularly within ±25°.

[0075] As is apparent from the result of FIG. 8A, when the propagationangle of light is in a range between ±35° and the sectional inclinationangle is in a range of from 28 to 43°, the exiting efficiency of thelight from the light guide plate can be made large.

[0076] However, if the sectional inclination angle is not larger than35°, the exiting angle of the light made to exit from thelight-guide-plate lower surface becomes large. This large exiting angleis the main cause of lowering of the contrast of the liquid-crystaldisplay device and of lowering of the frontal luminance of theliquid-crystal display device. That is, if the sectional inclinationangle is about 30°, the exiting angle of the light 3 made to exit fromthe light-guide-plate lower surface is in a range of from 18 to 38° (seeFIGS. 8B and 8C), the light 50 reflected by the light-guide-plate-1-sidesurface of the liquid-crystal display device 8 is apt to be produced asshown in FIG. 9A, so as to cause the lowering of the contrast. Further,the exiting angle of the display light 51 from the liquid-crystaldisplay device with respect to the liquid-crystal display device is aptto increase. Thus, the frontal luminance is lowered.

[0077] On the contrary, if the sectional inclination angle is about 40°,as shown in FIG. 9B, the exiting angle of the light 3 made to exit fromthe light-guide-plate lower surface is in a range of from 5 to 9° (seeFIG. 8B) or in a range of from 4 to 12° (see FIG. 8C). As a result, thelight 3 is difficult to be reflected by the light-guide-plate-1-sidesurface of the liquid-crystal display device 8 so that the contrast isenhanced. Further, since the exiting angle of the display light 51 fromthe liquid-crystal display device with respect to the liquid-crystaldisplay device is also reduced, the frontal luminance is enhanced.

[0078] When the sectional inclination angle is set to be not smallerthan 43°, the light exiting angle becomes small, but the light exitingefficiency is reduced as shown in FIG. 8A. This is because the totalreflection condition is not satisfied at the time of reflection by themicro-dot reflection slope 11. Accordingly, the light 2 entering theeyes of the observer directly from the light guide plate as shown inFIG. 2 increases undesirably to be the main cause of lowering of thevisibility.

[0079] From the above points, a preferable angle as the sectionalinclination angle is in a range of from 35 to 43° in which the lightexiting angle is small, the range of the exiting angle is narrow andthere is no reduction in the exiting efficiency.

[0080] Although the sectional inclination angle was set to be in a rangeof from 35 to 43° in this embodiment, it is more preferable that thesectional inclination angle is set to be in a range of 39 to 42° inwhich the range of the light exiting angle becomes minimal and opticaldesign becomes easy.

[0081] Incidentally, the propagation angle of the light propagatedinside the light guide plate 1 is generally about 35° when a cathode-raytube and a reflector are used as the light source 6. Alternatively, if alight emission diode is used as the light source 6, the propagationangle is approximately in a range between ±35° though it depends on thelens design of the light emission diode, and so on. It is thereforeimportant to determine a suitable sectional inclination angle inaccordance with the kind of the light source in consideration of theresults of FIGS. 8A to 8C and FIGS. 9A and 9B.

[0082] On the other hand, the reason why each of the micro-dots 5 isformed into a substantially V-shape in section is to reduce the haze(turbidity or cloudiness) of the light guide plate 1. Then, as shown inFIGS. 10A to 10C, respectively, the substantially V-shape includes aV-shape, a V-shape rounded at the vertex and a substantially U-shape,which are formed so that an R portion 20 of the micro-dot 5 is notlarger than about 20% of the dot depth (height) H14.

[0083] With such a sectional shape, slopes which do not contribute toreflection are reduced to the utmost (the area viewed from thelight-guide-plate upper surface 7 is small) when the light entering thelight guide plate is reflected by the reflection slopes.

[0084] It is preferable that the vertex angle of the sectional shape ofeach micro-dot 5 is set to be in a range of 70.6±2.5°. This is because amold for forming micro-dots in the light guide plate can be producedeasily by use of anisotropic etching technique of silicon single crystalas will be described later.

[0085] In addition, it is preferable that the plan shape of eachmicro-dot is formed to be substantially rectangular. Here, thesubstantially rectangular shape includes a rectangle and a rectanglerounded at its corners. Alternatively, the shape may be a square.Particularly, when the shapes of the micro-dots 5 are substantiallyrectangular, scattered light inside the light guide plate 1 is reducedso that the light exiting efficiency or the like is enhanced. At thesame time, the area of slopes which do not contribute to reflection inthe micro-dots 5 is reduced in comparison with that in the case of acircular shape or the like. As a result, there is an effect of reducingthe haze of the light guide plate.

[0086] Incidentally, although description was made in the aforementionedembodiment about the case where the micro-dots 5 were small recessportions, effects similar to those in the aforementioned case can beobtained even if each micro-dot has a protrusion portion as shown inFIG. 4, or a square pyramidal or wedge shape as illustrated in FIGS. 11Ato 11D.

[0087] Next, description will be made about the arrangement of themicro-dots.

[0088] The dot arrangement is preferably made so that the long-sidedirection of each dot 5 is substantially parallel with the longitudinaldirection of a cathode-ray tube or the like serving as the light source6 as shown in FIG. 12A. Alternatively, if a single point light sourcesuch as a light emission diode is used as the light source 6, it ispreferable that the long-side direction of each dot 5 is madesubstantially parallel with a tangential direction of a circle aroundthe point light source as shown in FIG. 12B. If a plurality of pointlight sources such as light emission diodes are used as the lightsources 6, the long-sidle direction of each dot 5 is made substantiallyparallel with a straight line connecting the plurality of point lightsources 6 as shown in FIG. 12C.

[0089] The reason is as follows. Most of the light beams entering thelight guide plate 1 from the light sources 6 travel substantiallyperpendicularly to the longitudinal direction of the light sources 6.After the light beams are incident on the slopes 11 of the micro-dots 5,they are reflected by the reflection slopes 11 to be thereby made toexit from the lower surface 10 of the light guide plate. Therefore, theabove-mentioned arrangement of the micro-dots is the most efficient.

[0090] Next, description will be made about the dimensions of themicro-dots 5.

[0091] Approximately, the short-side length W15 of each of themicro-dots 5 is, for example, in a range of from 0.002 to 0.05 mm, andthe long-side length L16 thereof is, for example, in a range of from theshort-side length to 0.2 mm, that is, in a range of from 0.002 to 0.2mm.

[0092] This is because, for example, in the case where original shapesof the micro-dots are formed by use of a well-known photolithographicmethod, it is difficult to form each of the micro-dots with a desirableprofile if the length of the micro-dot is not longer than 0.002 mm.

[0093] That is, the profiles of the micro-dots become irregular or thesurface accuracy of the dots in section is lowered due to the loweringof the resolution of a photo mask, the resolution of exposure ordevelopment, etc. As a result, the scattering of the light propagatedinside the light guide plate becomes so great that it becomes difficultto obtain a light guide plate which is high in light utilizationefficiency.

[0094] On the other hand, the reason why an upper limit of theshort-side length W15 of each of the micro-dots 5 is set to 0.05 mm isthat naked eyes have a visual limit of about 0.05 mm.

[0095] That is, if the short-side length of each of the micro-dots isnot smaller than 0.05 mm, the micro-dots themselves can be recognized,for example, even with naked eyes (there occurs a phenomenon that themicro-dots 5 can be seen with naked eyes, that is, a phenomenon that thelight guide plate looks like an aggregate of point light sources) sothat the visibility as the liquid-crystal display device using themicro-dots is deteriorated.

[0096] On the other hand, the reason why the long-side length of each ofthe micro-dots 5 is set to be not shorter than the short-side lengththereof is to increase the area of the reflection slopes 11 of themicro-dots. Thus, propagated light can be reflected effectively towardthe liquid-crystal display device without increasing the number ofmicro-dots 5 to be formed on the surface of the light guide plate 1.

[0097] In addition, the reason why the long-side length of each of themicro-dots 5 is set to be not longer than 0.2 mm is as follows. If thelong-side length of each micro-dot 5 is longer than 0.2 mm, themicro-dots 5 themselves can be recognized with naked eyes to therebyspoil the visibility as the liquid-crystal display device. Further, inthe case where density distribution is applied to the way of arrangementof the micro-dots 5, it is difficult to give a gradient to the densitydistribution if the long-side length of each of the micro-dots 5 is notshorter than 0.2 mm. As a result, uniform distribution of illuminationlight cannot be obtained.

[0098] Next, description will be made about the depth H14 of themicro-dot 5.

[0099] The depth H14 of each of the micro-dots 5 is definedautomatically based on the dot short-side length W15 and the sectionalinclination angle 12. It is, however, necessary to select the dotshort-side length W15 and the sectional inclination angle 12 so that thedepth H14 is in a range of from 0.002 to 0.04 mm. That is,, when the dotdepth H14 is not larger than 0.002 mm, the area of the reflection slope11 of the dot 5 becomes so small that the function of changing thetravelling direction of the light incident on the light guide plate 1 islost to reduce the utilization efficiency of light with which theliquid-crystal display device 8 is irradiated.

[0100] On the other hand, when the depth W14 of the dot 5 is set to benot smaller than 0.04 mm, the quantity of irradiation from the lightguide plate 1 increases in an area near the light source 6, so thatuniform irradiation becomes difficult.

[0101] Next, description will be made about the arrangement of themicro-dots 5.

[0102] In conclusion, it is desirable that the micro-dots 5 are arrangedwith no regularity. This is because the micro-dots 5 described in thisembodiment are extremely minute so that the irregular arrangement of themicro-dots 5 is necessary for prevention of a moire phenomenon fromoccurring due to the interference of the micro-dots 5 with a regularlyformed pattern of a member constituting the liquid-crystal displaydevice 8, for example, represented by a liquid-crystal cell, a colorfilter, a TFT pattern, a black matrix, etc.

[0103] Particularly, when the illuminator is disposed in front of theliquid-crystal display device 8 in use, there is no diffusing plate,which would be often used on normal occasions, between the light guideplate 1 and the observer. Therefore, it is an extremely importantproblem to prevent such a moire phenomenon.

[0104] However, when the micro-dots 5 are arranged simply withirregularity, it is easy to produce a cluster 21 of micro-dots 5 or anarea 22 where there is no micro-dot 5, as illustrated in FIG. 13. As aresult, the visibility as the liquid-crystal display device may bedeteriorated.

[0105] It is therefore preferable that a radial distribution functionsimilar to the case described in JP-A-10-1537519 is used to form themicro-dots so as to satisfy the following conditions.

[0106] That is, in the light guide plate 1, the surface on which themicro-dots 5 are formed is sectioned into 0.25 to 1 mm² square areasover an area not smaller than 95% of the aforementioned surface on whichthe micro-dots 5 are formed. In each of the square areas, the dots 5 areformed and disposed so that a function G(R) which is obtained by takinga weighted average of a radial distribution function g(R) obtained foreach dot in accordance with the arrangement relationship of the dots,and which is obtained by approximating the weighted average by a leastsquares method satisfies the relation of 0<S₁/S₂<0.2 in a range ofR/R₀=3 to 6;

[0107] provided that R designates a distance from a central position ofone dot to a central position of another dot; R₀, a value obtained bydividing a length of one side of the square area by a square root of thenumber of the dots existing in the square area; S₁, a value obtained byintegrating a difference between G(R) and an average value of G(R) withR/R₀ which is in a range of from 3 to 6; and S₂, a value obtained byintegrating the average value of G(R) with R/R₀ which is in a range offrom 3 to 6.

[0108] The reason why the aforementioned arrangement is required in anarea not smaller than 95% of the surface on which the micro-dots 5 areformed is that a measure to prevent moire is required in theaforementioned area because the dots themselves may be observed directlywhen the illuminator is disposed in front of the liquid-crystal displaydevice. Thus, the visibility as the liquid-crystal display device can beensured.

[0109] The area of each square is determined to include, preferably, atleast 10 micro-dots 5, more preferably at least 50 micro-dots 5 in thesquare area. That is, if the area of the square is not larger than 0.25mm², the number of dots included in the square area is too small tocalculate the radial distribution function g(R) because the value of Rois usually approximately in a range of from 0.01 to 0.2 mm.

[0110] On the contrary, if the area of the square is set to be notsmaller than 1 mm², the quantity of the light radiated from the lightguide plate 1 cannot be estimated correctly when the dot densitydistribution is changed to correct the light quantity. Thus, it may bedifficult to correct the light quantity. TABLE 1 Relationship betweenS1/S2 Value and Moire Occurrence item Absolute random-number arrangementmethod overlap no no 0.02 mm 0.02 mm 0.03 mm 0.04 mm constraint S1/S20.8 0.5 0.5 0.3 0.2 0 moire ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ dot overlap x x x Δ ∘ ⊚dark/bright x x ∘ ∘ ∘ ⊚ Spot total x x x Δ ∘ ⊚ valuation

[0111] Table 1 shows the results of investigation about the relationshipbetween the production of moire and the aforementioned coefficients,while the range of the value S₁/S₂ was determined on the basis of theresult described above. Further, the function G(R) is set to besubstantially 0 in the range of R<(short-side length of dot)×2. Thissetting is to prevent dot overlap, which is produced when dots are closeto each other, from being observed.

[0112] Incidentally, the overlap constraint in Table 1 is a method fordefining a shortest distance between dots adjacent to each other so asto make the function G(R) substantially 0 in the range of R<(short-sidelength of dot)×2.

[0113] Since the dot short-side length is 0.01 mm in this embodiment,the function G(R) can be made substantially 0 in the range ofR<(short-side length of dot)×2 if the shortest distance between dotsadjacent to each other is set to be not shorter than 0.02 mm. The “spot”is that which is obtained by judgement as to whether a dark area wheredots overlap with each other or a bright area where there is no dot isgenerated and can be visually confirmed or not.

[0114] Further, FIG. 14 shows the relationship between the function G(R)and the distance R. As is apparent from this result, the aforementioneddots are arranged so that at least two peaks each of which is at leasttwice as large as the average value of the function G(R) exist in thefunction G(R) in a range of R/R₀=3 to 6. The reason for such dotarrangement is that, in the case where substantially rectangular dotsare used, the dot density is easily enhanced if the dot interval on theshort side is made shorter than the dot interval on the long side.

[0115] On the other hand, the reason why each of the peaks is made atleast twice as large as the average value of the function G(R) in arange of R/R₀=3 to 6 is that, by adding such a condition, the distance(position relationship) between dots adjacent to each other can be keptsubstantially constant so that the production of a cluster of dots orthe production of an area where there is no dot can be prevented.

[0116] Next, description will be made about the method for manufacturingthe light guide plate according to this embodiment.

[0117] Fundamentally, a mold is first manufactured and, the light guideplate is then manufactured by performing plastic molding with the mold.At this time, as the method for manufacturing the mold, variouswell-known machining methods, such as drilling, cutting, grinding, etc.,may be used. Alternatively, an electrical discharge machining method isalso effective.

[0118] The number of micro-dots is in a range of from 200 to 20,000pieces/cm² in this embodiment, and hence it exceeds 1,000,000 pieces intotal on the whole surface of the light guide plate. Therefore, itshould be, however, regarded as very difficult to form such a largenumber of micro-dots by the aforementioned manufacturing method.

[0119]FIGS. 15A to 15G show a process of forming a mask pattern on asilicon substrate, and FIGS. 15H-15M show a process of formingmicro-dots by use of an anisotropic etching method.

[0120] This manufacturing method has the steps of:

[0121] (A) cutting a silicon substrate 30 out of a silicon singlecrystal ingot 29 so that the silicon substrate 30 has a predeterminedcrystal plane;

[0122] (B) forming a silicon oxide film 31 on the surface of the siliconsubstrate 30 by use of a well-known method;

[0123] (C) forming a photo-resist film 32 on the silicon oxide film 31;

[0124] (D) disposing a photo mask 33 having a micro-dot pattern on thesilicon substrate 30, and irradiating the resist film 32 withultraviolet rays (UV) from above the mask 33 to thereby expose theresist film 32;

[0125] (E) developing the resist film 32, and forming a pattern 34 ofmicro-dots on the silicon oxide film 31;

[0126] (F) pasting a protective tape 35 on the silicon oxide film 311which is formed on the back surface of the silicon substrate 30, andremoving the silicon oxide film 31 from portions of the siliconsubstrate 30 other than the back surface by a well-known etching method;

[0127] (G) removing the resist film 32;

[0128] (H) anisotropically etching the silicon substrate 30 with thepattern 34 of the silicon oxide film 31 serving as a mask;

[0129] (I) removing the protective tape 35 and the silicon oxide film31;

[0130] (J) forming a plating undercoat film 36 on the etched surface ofthe silicon substrate 30 by use of a well-known method;

[0131] (K) forming a plating film 37 by use of a well-known platingmethod with the plating undercoat film 36 serving as an electrode;

[0132] (L) stripping the plating film 37 off, and producing a stamper 38having the micro-dot pattern 34;

[0133] then, performing abrasion on the micro-dot surface of the stamper38 and the back surface thereof in accordance with necessity(not-shown); and

[0134] (M) installing the stamper 38 in a well-known molding machine,and forming a light guide plate 1 by an injection molding method.

[0135] The respective steps will be described below in detail.

[0136] First, the step of cutting the silicon substrate 30 out of thesilicon single crystal ingot 29, as shown in the step (A), is one of themost important steps in the manufacturing method.

[0137] When micro-dots each having a substantially V-shape in sectionare formed on the surface of the light guide plate 1, differences inetching speed in accordance with crystal orientations of siliconcrystals as shown in Table 2 are utilized. That is, even if etching isperformed on a crystal with any crystal plane as the silicon substrate30, a (111) plane for which the etching speed is lowest is finally setto have reflection slopes of the micro-dots formed. TABLE 2 EtchingSpeed of Anisotropic Etching Etching speed Crystal plane (m/sec) 1001.05E-05 110 2.15E-05 210 2.06E-05 211 1.64E-05 221 9.77E-06 3101.80E-05 311 1.78E-05 320 2.14E-05 331 1.41E-05 530 2.12E-05 5402.14E-05 111 1.50E-07

[0138] By utilization of such a crystal characteristic, it is possibleto form micro-dots each of which has an optional sectional inclinationangle and has a substantially V-shape in section and the plan shape ofwhich is substantially rectangular.

[0139]FIG. 16A is a view showing a method for cutting out a crystal whenthe sectional inclination angle is 40° In this embodiment. FIG. 16B is aschematic view of the silicon substrate 30 cut out. FIG. 17 is a planview of the silicon substrate 30 cut out.

[0140] By performing anisotropic etching on this silicon substrate 30,it is possible to form micro-dots having a predetermined sectionalinclination angle shown in FIG. 16B by way of example. Here, by changingthe angle with which the silicon substrate 30 is cut out, the siliconsubstrate 30 can be manufactured to have an optional crystal plane. As aresult, as described above, it becomes possible to manufacture thesilicon substrate 30 having a desirable sectional inclination angle inaccordance with the kind of light source. In addition, the crystal planeof the silicon single crystal is not limited to the (001) plane. Asilicon single crystal having an optional crystal plane may be used.Further, a silicon single crystal with a desired crystal planedetermined at the beginning may be manufactured and used.

[0141] For the step of forming the silicon oxide film 31 which is usedas a mask in anisotropic etching on the surface of the silicon substrate30, as shown in the step (B), various methods may be used. In thisembodiment, al well-known thermal oxidation method was used. Table 3shows an example of thermal oxidation conditions. TABLE 3 Example ofAnisotropic Etching Process Step Conditions  1 Forming thermally-temperature: 1,000° C., water oxidized film on Si temperature: 90° C.wafer film thickness: 0.0005 mm, 31 min  2 Baking before resist nitrogenatmosphere, 140° C. 30 min coating  3 Photo-resist coating OFPR-8600 10cp 1,000 rpm film thickness: 0.001 mm  4 Pre-baking nitrogen atmosphere,90° C. 30 min  5 Exposure/development exposure: 50 mJ developer: NMD-3 6 Post-baking nitrogen atmosphere, 140° C. 30 min  7 Oxygen plasmaashing 800W 400 sccm 3 min 45 s  8 Oxide film etching dip HF: NH₄F = 1:7etching time: 6 min  9 pasting protective film made by Nitto Electricfilm on back surface Industrial Co., Ltd. 10 removing photo-resist S502Astripping liquid 110° C. 10 min 11 Si anisotropic etching 20 wt % KOH1.5 hr 12 removing oxide film dip HF: NH₄F = 1:7 etching time: 6 min 13washing/drying vapor washing: 5 min 14 forming thermally- Temperature:1,000° C., water oxidized film again temperature 90° C. film thickness:0.001 mm, 60 min

[0142] Next, in the step (C) of forming the photoresist film 32 on thesilicon oxide film 31, it is preferable that a primer is applied to thesilicon oxide film 31. as a pre-process so as to improve the adhesion toan undercoat film. As a proper method for the primer treatment, variousmethods may be used. For example, when a silane agent is used as theprimer, hexamethylsilazane is suitable. That is, a so-called gaseousdiffusion process is used so that the hexamethylsilazane is supplied toa vessel and evaporated to form a thin film on the substrate surface.Thus, a uniform film can be formed on the silicon oxide film 31.

[0143] As the photo-resist material, for example, a fluid-like orfilm-like positive type or negative type material may be used. In FIG.15C, a positive type material was formed by use of a well-knownspin-coating method.

[0144] In the step (D), for example, a chromium mask, a film mask, anemulsion mask, etc. may be used as the photo mask. Data such as the sizeand number of designed micro-dots, the distribution thereof, and so on,are prepared in advance. A pattern of the micro-dots is drawn, forexample, by use of an electron beam method, a laser beam method, or thelike. This pattern is used as the mask.

[0145] In the steps (E) to (G), exposure, etching of the silicon oxidefilm 31, and removal of the resist film 32 are performed by well-knownmethods respectively.

[0146] By the above steps, the silicon substrate 30 having apredetermined micro-dot pattern in the silicon oxide film 31 iscompleted.

[0147] Next, as shown in the step (H), anisotropic etching is performedon the silicon substrate 30 with the pattern of the silicon oxide film31 serving as a mask. Table 3 shows an example of etching processconditions. A KOH solution the KOH concentration of which was about 20%was used as etching liquid. In such conditions, micro-dots having aV-shape in section with a sectional inclination angle of about 40° wereformed on the surface of the silicon substrate 30.

[0148] Succeedingly, in the step (I), the protective tape 35 formed onthe back surface of the silicon substrate 30, and the silicon oxide film31 are removed by use of a well-known method. Then, a plating layer(stamper) is formed by a plating method shown in the steps (J) and (K).Incidentally, if the undercoat film 36 is formed on the siliconsubstrate 30 having the micro-dots 30 in advance, unevenness of theplating film can be reduced in the plating step so that a superiorplating layer, that is, a superior stamper can be formed.

[0149] Although the aforementioned undercoat film may be formed by useof a well-known plating method or may be formed of a sputter film suchas an Ni thin-film or the like, this film thickness is an extremelyimportant parameter. That is, if the film thickness is large, therearises a problem that the thin film is stripped off in the plating step.

[0150] Therefore, in this embodiment, the film thickness was controlledto be in a range of from 0.015 to 0.035 μm, especially in a range offrom 0.02 to 0.03 μm. If the film thickness is out of this range, therearises a problem that uniform plating processing becomes impossible (ifthe undercoat film thickness is thin), or the undercoat film 36 or theplating film 37 which is formed with a micro-dot pattern is stripped off(if the undercoat film thickness is thick).

[0151] Although various metals may be used as the material for theundercoat film 36 and the plating layer 37 formed by the plating method,Ni material was used here in consideration of uniformity of filmthickness and mechanical performance.

[0152] Next, as shown in the step (L), the obtained plating film 37 isstripped off from the silicon substrate 30 so as to be used as thestamper 38 for forming micro-dots in the surface of the light guideplate. At this time, in order to obtain a light guide plate with a highlight utilization efficiency, it is important to perform abrasion on themicro-dot surface. Therefore, abrasion was performed with aluminaabrasive grains the average grain size of which was in a range of from0.1 to 1 μm in this embodiment. However, it is not limited to the aboveabrasion, hand lapping or machine lapping with diamond abrasive grainsmay be performed.

[0153] Finally, as shown in the step (M), for example, the obtainedstamper is fixed to a matrix of an injection molder by a magnet, avacuum chuck, or the like, and a material to form the light guide plateis supplied to the matrix. Thus, the light guide plate having micro-dotswith predetermined dimensions is completed. Incidentally, extrusionmolding, compression molding, vacuum molding, or the like, which areknown well, may be used as the molding method.

[0154] General transparent plastic materials are available as thematerial to form the light guide plate. Specific examples of availablematerials include acrylic plastic, polycarbonate resin, polyacetalresin, polyolefin resin, ultraviolet-curing plastic material.Particularly, since acrylic resin material is superior in transparency,price, moldability, and so on, it is a material suitable formanufacturing the light guide plate according to this embodiment.

[0155] Next, description will be made about a second embodiment wherethe aforementioned light guide plate has been applied to aliquid-crystal display device, with reference to FIG. 18.

[0156]FIG. 18 is a schematic sectional view of a liquid-crystal displaydevice. On the lower surface of the light guide plate 1, there aredisposed a polarizer 40, a phase-difference film 41, a diffusing film42, a glass substrate 43, a color filter 44, a pixel electrode 45, aliquid-crystal cell 46, a TFT 47, a reflection polarizer 48, anabsorbing film 48, etc. This configuration shows an example of awell-known reflection type liquid-crystal display device. Variousconfigurations may be considered in accordance with applications ofliquid-crystal display devices.

[0157] Specific examples of a light source 6 include a cathode-ray tube,a light emission diode, an EL element, etc. as mentioned above. Asuitable light source is selected from the point of view of powerconsumption, use form, etc. In this embodiment, five light emissiondiodes were used. In addition, optical parts including theliquid-crystal cell 46 are not limited specifically. Well-known partswere used for the optical parts.

[0158] The light guide plate measured about 30×30×1 mm. Each ofmicro-dots 5 which is formed on the surface of the light guide plate 1measured 0.01 mm in dot short-side length, 0.08 mm in dot long-sidelength, 40° in sectional inclination angle, and 70.6° in vertex angle.Particularly, the sectional inclination angle was set to be 40° so as tomake the light divergent angle in a range of about ±25° and so as torestrain the divergence of the exiting angle from the light guide plate1 to be small because the light sources 6 were made of light emissiondiodes (see FIGS. 8A to 8C and FIGS. 9A and 9B).

[0159]FIG. 19 is a view conceptually showing a section of aliquid-crystal display device in front of which the aforementionedilluminator has been disposed. Other than the illuminator 23 disposed infront of the liquid-crystal display device 8, there are provided adriving circuit 26 for driving the liquid-crystal display device 8, acontrol circuit 24 and a power supply 25 for driving the liquid-crystaldisplay device 8 and the driving circuit 26.

[0160] Under a normal usage environment, for example, when theliquid-crystal display device 8 is used indoors or outdoors withsufficient external light, display showing sufficient luminance can beperformed without using the illuminator 23. If the light quantity fromthe external light is insufficient, the illuminator 23 is controlled bythe control circuit 24 so that required light can be supplied from thelight sources to the liquid-crystal display device 8.

[0161] When the liquid-crystal display device was irradiated with thelight from the light sources in addition to the external light with theabove configuration, the liquid-crystal display device exerted muchhigher visibility than a background-art liquid-crystal display devicehaving no light guide plate. Thus, high luminance display could beattained. Since it will go well if the illuminator 23 is operated inaccordance with necessity, the power consumption can be reduced whilevisibility required of the liquid-crystal display device is ensured.

[0162]FIG. 20 is a conceptual view for explaining a third embodiment.

[0163] In this embodiment, a portable electronic apparatus in which theilluminator 23 has been disposed in front of the liquid-crystal displaydevice 8 is illustrated. In addition to the configuration of theembodiment shown in FIG. 20, the portable electronic apparatus has alight receiving element 27 disposed on the side facing theliquid-crystal display device 8. Specifically, a mobile liquid-crystaldisplay device, a portable telephone, or the like, is assumed.

[0164] In this case, for example, external light, for example, sunlightor the like, is received also in the light receiving element 27, and anelectric signal converted in the light receiving element 27 is suppliedto the control circuit 24. Then, the control circuit 24 controls theilluminator 23 in accordance with the magnitude of this electric signalso as to adjust the quantity of light with which the liquid-crystaldisplay device 8 is irradiated.

[0165]FIG. 21 is a view conceptually showing the relationship betweenthe quantity of external light and the luminance of the liquid-crystaldisplay device. The abscissa designates that the quantity of externallight increases as the abscissa goes toward the left side. As isapparent from this drawing, when the quantity of external light islarge, the frontal luminance of the liquid-crystal display device can beensured sufficiently so that the illuminator does not have to beoperated.

[0166] However, as the quantity of external light decreases, the frontalluminance of the liquid-crystal display device is lowered. In thisstate, sufficient frontal luminance of the liquid-crystal display devicecannot be ensured so that the visibility of display is remarkablylowered.

[0167] In such a case, when the illuminator is operated in accordancewith the operation curve of the illuminator as illustrated in FIG. 21,sufficient visibility of display can be attained even if the quantity ofexternal light is insufficient. In addition, the liquid-crystal displaydevice can be controlled to make display with substantially constantluminance regardless of the quantity of external light.

[0168] As has been described above, according to this embodiment, lightcan be automatically supplied from the illuminator in accordance withthe quantity of external light. Accordingly, the visibility of theliquid-crystal display device can be enhanced while the convenience ofbeing a portable apparatus is ensured.

[0169] Incidentally, the aforementioned embodiment is only an example,and not to say, the present invention is not limited to the embodiment.For example, a receiving terminal or a receiving device for receiving aninformation signal may be further provided so that, when an informationsignal is received, the control circuit controls the illuminator(including the light receiving element) with this signal as a trigger tothereby adjust the frontal luminance of the liquid-crystal displaydevice. Alternatively, an observer may use a switch or a volume to beable to adjust the frontal luminance of the liquid-crystal displaydevice as the occasion demands.

[0170] As has been described above, the luminance of a display screencan be enhanced by disposing an illuminator according to the presentinvention in front of a liquid-crystal display device. In addition, alight guide plate which is high in light utilization efficiency can bemanufactured by using an anisotropic etching method on a siliconsubstrate having a predetermined crystal plane.

[0171] While we have shown and described several embodiments inaccordance with our invention, it should be understood that disclosedembodiments are susceptible of changes and modifications withoutdeparting from the scope of the invention. Therefore, we do not intendto be bound by the details shown and described herein but intend tocover all such changes and modifications a fall within the ambit of theappended claims.

We claim:
 1. An illuminator disposed in front of a liquid-crystal cell,comprising: a light guide plate; and a light source disposed on one ofside surfaces of said light guide plate; wherein said light guide plateincludes an incidence surface on which light from said light source isincident,, and a light transmission surface from which said incidentlight is made to exit to said liquid-crystal cell; and wherein aplurality of dots each constituted by a small recess or protrusionportion for reflecting said light from said incidence surface towardsaid light transmission surface are formed in or on another surface ofsaid light guide plate opposite to said light transmission surface, eachof said dots having a substantially V-shape in section, an inclinationangle of said section being in a range of from 35 to 43°.
 2. Anilluminator disposed on a front surface of a liquid-crystal cell,comprising: a light guide plate; and a light source disposed on one ofside surfaces of said light guide plate; wherein said light guide plateincludes an incidence surface on which light from said light source isincident, and a light transmission surface from which said incidentlight is made to exit to said liquid-crystal cell; and wherein aplurality of dots each constituted by a small recess or protrusionportion for reflecting said light from said incidence surface towardsaid light transmission surface are formed in or on another surface ofsaid light guide plate opposite to said light transmission surface, anarea not smaller than 95% of a whole area of the surface where said dotsare formed is sectioned into 0.25 to 1 mm² square areas, and said dotsare disposed in each of said square areas so that a function G(R) whichis obtained by taking a weighted average of a radial distributionfunction g(R) obtained for each of said dots in accordance with anarrangement relationship of said dots and which is obtained byapproximating said weighted average by a least squares method satisfiesa relation of 0<S₁/S₂<0.2 in a range of R/R₀=3 to 6; provided that Rdesignates a distance from a central position of one dot to a centralposition of another dot; R₀, a value obtained by dividing a length ofone side of said square area by a square root of the number of said dotsexisting in said square area; S₁, a value obtained by integrating adifference between G(R) and an average value of G(R) with R/R₀ which isin a range of from 3 to 6; and S₂, a value obtained by integrating saidaverage value of G(R) with R/R₀ which is in a range of from 3 to
 6. 3.An illuminator according to claim 1, wherein: a vertex angle of each ofsaid dots is in a range of 70.6±2.5°.
 4. An illuminator according toclaim 1, wherein: each of said dots is substantially rectangular in planshape and has a short-side length in a range of from 0.002 to 0.05 mmand a long-side length in a range of from 0.002 to 0.2 mm.
 5. Anilluminator according to claim 1, wherein: said dots are disposed atrandom in or on the other surface of said light guide plate opposite tosaid light transmission surface.
 6. An illuminator according to claim 2,wherein: each of said dots is disposed so that said function G(R) issubstantially 0 in a range of R<(short-side length of said dot)×2, atleast two peaks exist in said function G(R), and said two peaks each ofwhich said two peaks is at least twice as large as said average value ofsaid function G(R) exist in a range of R/R₀=3 to
 6. 7. A method formanufacturing a light guide plate having dots, comprising the steps of:(a) producing a silicon substrate having a predetermined crystal plane;(b) forming an oxide film on a surface of said silicon substrate; (c)forming a resist film on said oxide film, and forming a dot pattern insaid oxide film by using said resist film as a mask; (d) anisotropicallyetching said silicon substrate by using said oxide film as a mask; (e)forming a metal film on said silicon substrate; (f) stripping said metalfilm off so as to produce a stamper or a replica thereof; and (g)transferring said dots onto a surface of a film or a plastic sheet orplate by using said stamper or said replica.
 8. A method formanufacturing a light guide plate according to claim 7, wherein: acrystal plane of said silicon substrate is selected so that each of saiddots formed in or on said light guide plate has a V-shape in section andan inclination angle of said section is in a range of from 35 to 43°. 9.A liquid-crystal display device comprising: an illuminator; aliquid-crystal display portion; and a control portion; wherein saidilluminator is disposed in front of said liquid-crystal display portionso that external light is transmitted through said illuminator andenters said liquid-crystal display portion, and said illuminator iscontrolled in accordance with a quantity of said external light by usingsaid control portion.
 10. A liquid-crystal display device according toclaim 9, wherein: when a quantity of said external light entering saidliquid-crystal display portion is not larger than a predetermined value,said illuminator is controlled by said control portion so as to increasethe quantity of light entering said liquid-crystal display portion. 11.A portable electronic apparatus comprising: an illuminator; aliquid-crystal display portion; a light receiving portion; and a controlportion; wherein said illuminator is disposed in front of saidliquid-crystal display portion so that external light is transmittedthrough said illuminator and enters said liquid-crystal display portion,and said illuminator is controlled by using a quantity of said externallight received by said light receiving portion, so that luminance ofsaid liquid-crystal display portion is made substantially constant. 12.A portable electronic apparatus comprising: an illuminator; aliquid-crystal display portion; a light receiving portion; a signalreceiving portion; and a control portion; wherein said illuminator isdisposed in front of said liquid-crystal display portion so thatexternal light is transmitted through said illuminator and enters saidliquid-crystal display portion, and said illuminator is controlled bysaid control portion by using a signal supplied to said signal receivingportion as a trigger so that said liquid-crystal display portion isirradiated with light in accordance with a quantity of said externallight entering said light receiving portion and so that luminance ofsaid liquid-crystal display portion is made substantially constant.